The 22-tonne core stage of a Chinese rocket is expected to fall back to Earth sometime on Saturday, the third time in two years that China has allowed such a massive booster to re-enter the atmosphere unchecked. Unguided re-entry poses a small but avoidable risk to the world’s population, space debris experts have said.
The Long March 5B rocket lifted off on July 24 with the Wentian module for China’s Tiangong space station, carrying one of the heaviest payloads launched into orbit in recent years. The nearly 100-foot-long (30-meter) core stage of the Long March 5B rocket fired its two hydrogen engines for about eight minutes to inject the Wentian module into orbit.
Four strap thrusters burned their propellant and jettisoned minutes after launch to fall into the South China Sea. But the design of the Long March 5B, one of the most powerful operational rockets in the world, means that its core stage accelerates to orbital speed.
Most launch vehicles carry an upper stage to complete the job of putting a payload into orbit, letting the propellant fall back to Earth in the ocean or be picked up for reuse, as SpaceX does with its Falcon 9 rocket.
On Saturday morning, the Long March 5B core stage was expected to re-enter the atmosphere between 4:15 p.m. GMT (12:15 p.m. EDT) and 6:15 p.m. GMT (2:15 p.m. EDT), according to a forecast from the Aerospace Corp., a California-based, federally funded, non-profit research institute.
The rocket’s orbit is between 41.5 degrees north and south latitude during each 1.5-hour lap around the Earth. The land between these latitudes is home to approximately 88% of the world’s population.
“It’s a low risk globally, but it’s an unnecessary risk, and it can affect people, which is why we’re talking about it,” said Ted Muelhaupt, consultant at Aerospace Corp. and expert on space debris reentry.
It’s impossible to predict exactly when or where the rocket will re-enter the atmosphere, but the surviving debris will likely fall into a long, narrow footprint hundreds of miles long and up to tens of miles across. Rocket wreckage will most likely fall into the ocean or unpopulated areas.
This is the third time that China has left a Long March 5B core stage in orbit to return to Earth unguided. The uncontrolled re-entry of the central first stage of Long March 5B in 2020 spread debris across Côte d’Ivoire. The reentry of Long March 5B last year occurred over the Indian Ocean, and no debris was found.
The window of uncertainty around when the rocket will re-enter the atmosphere is largely due to unknowns about the orientation of the rocket and the ever-changing density of the upper atmosphere, which is driven by solar activity. which causes the atmosphere to expand or contact, according to Muelhaupt.
The window is shrinking as the time of re-entry approaches. Five days before the start of the school year, the experts estimated the window with an error of plus or minus one day. Saturday morning, only a few hours before the start of the school year, the error was reduced to more or less an hour.
The aerodynamic drag will eventually slow the rocket’s speed enough to allow Earth’s gravity to retreat into the atmosphere, where most of the booster stage will burn up. Muelhaupt estimates that about 4 to 9 metric tons, or 20% to 40% of the rocket’s dry mass, will survive the scorching heat of reentry and reach Earth’s surface.
Abandoned rocket bodies and dead satellites regularly re-enter the atmosphere. About 50 man-made objects weighing more than a ton enter the atmosphere out of control every year, according to Muelhaupt.
But the Long March 5B core stage will be the sixth-largest object to re-enter the atmosphere, not counting the space shuttle, Muelhaupt said.
The Aerospace Corp. estimates that there is a 1 in 230 to 1 in 1,000 chance that a piece of the Long March 5B center stage will kill or injure a person, meaning there is a 99.5% chance that there are no victims of the re-entry.
But U.S. government policy guidelines ask space mission managers to ensure that the risk of death or injury during reentry does not exceed 1 in 10,000. The risk of harm from reentry from the Long March 5B is estimated to be at least 10 times the standard risk threshold for US space missions.
“When it drops, it will definitely exceed the 1 in 10,000 threshold which is the generally accepted guideline,” Muelhaupt said. “And one of the reasons we’re paying particular attention to that is that in May 2020, the first test launch of that dropped debris in Africa.”
The risk of re-entry for a single person is even lower — 6 trillion out of 10, according to Aerospace Corp’s assessment.
“The reality is that there are a number of things you can do about this type of thing, particularly if you think ahead with your mission,” said Marlon Sorge, executive director of the Center for orbital debris and aerospace re-entry studies.
For example, designers can select materials that are more likely to burn up on re-entry, reducing the chance of debris surviving to hit the Earth’s surface.
“With rocket bodies, they’re so big that what you do in your design phase doesn’t really matter in terms of what you do with them. You have huge pieces of metal where the engines are,” Sorge said.
“But there are other approaches you can do if you’re thinking about the lead, and one of them is controlled re-entry,” Sorge said. “Basically, once you’ve finished delivering your payload, you spin your rocket, fire up the engine, and take it back out into the ocean somewhere, usually, somewhere where there’s no population. You do that, and you’ve pretty much mitigated the risk here. And that’s one of the things that are being done by the US government to mitigate those types of risks.
After the last launch and re-entry of Long March 5B last year, NASA Administrator Bill Nelson said China “failed to meet responsible standards for their space debris”.
“Space nations must minimize the risks to people and property on Earth from space object reentries and maximize transparency regarding these operations,” Nelson said in a statement last year.
Wang Wenbin, spokesperson for China’s Foreign Ministry, told a press conference last year that it was “standard practice” for the upper stages of rockets to burn up on re-entry into the atmosphere. He incorrectly referred to the body of the Long March 5B rocket as an upper stage and said “most of its parts will burn out on reentry, making the likelihood of damage to air or ground facilities and activities extremely low.” “.
But no other launcher in the world leaves such a massive component in orbit to fall back to Earth. Dead satellites and old rocket stages regularly re-enter the atmosphere, but re-entering objects with masses of more than a few tons are rare.
“Why are we worried? Well, it caused property damage last time (a Long March 5B came in),” Muelhaupt said this week. “People have to prepare accordingly.
“And besides, it’s not necessary,” he said. “We have the technology to not have this problem. Whenever you see a Falcon 9 land, this main stage will not drop somewhere randomly. Deliberately dropping objects into the ocean, when they are large enough to cause damage, is the practice we would like to encourage.
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